The presence of particulate matter, such as soot particles, in the environment has brought about an increased interest in the development of methods and devices for the determination of particulate concentration. Soot in particular has been the subject of study for measurement. However, all small particles pose an important area of interest and concern, particularly for environmental and health reasons. The emission of soot from engines, power generation facilities, incinerators, or furnaces, for example, represents a loss of useful energy and further is a serious environmental pollutant and a health risk. However, the presence of soot in flames can also have positive effects. For example, the energy transfer from a combustion process is largely facilitated by the radiative heat transfer from soot. Thus, to understand soot formation and develop control strategies for soot emission or formation, measurements of soot concentrations are necessary. Laser Induced Incandescence (LII) is a good diagnostic tool for measurements of particulates as the LII signal is proportional to particle volume fraction.
The measurement of soot particle concentrations has been greatly improved by the development of Laser Induced Incandescence (LII), which can provide concentration information with high temporal and spatial resolution. Previous techniques could not detect small concentrations and could not provide accurate time responsive information regarding soot formation.
LII exposes a volume of gas containing refractory particles, that is particles capable of absorbing laser light energy with an evaporation temperature sufficiently high to produce measurable incandescence, to a pulsed focused high-intensity laser light. The particles absorb laser energy heating to temperatures far above the surrounding gas. At these elevated temperatures (about 4000-4500 K in the case of soot) the particles incandesce strongly throughout the visible and near infrared region of the spectrum. The maximum temperature is controlled by the point at which evaporation becomes the predominant heat loss mechanism. Any further increase in laser light energy then tends to result in an increase in the evaporation rate rather than an increase in particle temperature. In accordance with Planck's radiation law, the radiative emission at these elevated temperatures increases in intensity and shifts to blue wavelengths. Thus the LII signal is readily isolated from any natural flame emission. Because of the rapid time scale and good spatial resolution, as well as its large dynamic range, LII is well suited as an optical diagnostic to measure soot volume fraction in turbulent and time varying combustion devices.
In an application by Alfred Leipertz et al WO 97/30335 published Aug. 21, 1997 a laser-induced incandescence technique is described for determining a primary particle size. The method taught by Leipertz includes the measurement of the incandescence at two discrete points in time after the laser light pulse, from which a ratio is generated to calculate the particle size according to a mathematical model. However, this method has been shown to be prone to inaccuracies. Leipertz samples the two measurements at a point of decay where he assumes a linear change. This is not possible until significant cooling has occurred and most of the signal has passed. Thus the signals measured by Leipertz are very weak and are highly influenced by noise. And the assumption of a linear decay in the incandescence is not accurate adding to the inaccuracy of the system. The ambient temperature of the surrounding gas is also significant to modeling the decay, and is not considered by Leipertz. Laser fluence over the volume measured is also critical to the subsequent decay. It is critical for accuracy to know the energy density profile over the volume. This factor is assumed without verification by Leipertz's technique. Further error is introduced by the detection method which uses broad band detectors to measure the signal. Since the detected incandescence intensity is used as an indication of temperature, and the intensity varies in accordance with temperature and wavelength, a sample over a broad band of wavelengths greatly complicates and obscures accurate measurement. The Leipertz technique, as a result of these introduced errors, does not provide a good measurement of particle size.
Attempts to characterize particle size are also disclosed in a paper "Soot diagnostics using laser-induced incandescence in flames and exhaust flows" by R. T. Wainner and J. M. Seitzman published in 1999, by the American Institute of Aeronautics and Astronautics. This article reviews a method to determine particle size by measuring the peak temperature attained (pyrometry) by LII. However, the study found that the temperature of different-sized particles can be identical and thus temperature measurement at the peak is not sufficient to determine particle size.
Thus an accurate method for particle size is still needed. In addition, the known LII techniques as currently practiced are not practical for use in diagnostic and emissions testing of combustion engines. A compact and portable device is needed for practical use. Current methods for measuring diesel particulates are the Bosch Smoke Number and the direct mass sampling. In the Bosch Smoke Number method particulates are collected on filter paper from a portion of the exhaust stream and the light reflection from the collected sample is measured. This is compared against a calibration chart to determine the mass flow. Since sufficient sample material must be collected over time, this method requires a long period for sample collection and has a poor time and spatial resolution. Thus this method cannot provide diagnostic information about the formation of particles in the combustion cycle. The direct mass sampling method is the official method of the EPA and measures the mass of soot from a difference of the mass of the soot on a filter and subtracting the mass of the filter. This method, however, has a limited accuracy, particularly for low emission vehicles. Both methods suffer a loss in accuracy when the source produces lower emissions, and require significantly longer testing for low emission combustors.
It is desired to accurately measure the primary particle size of particles with high temporal and spatial resolution. Small particles, in particular, have been found to present significant health concerns. However, using traditional methods particles under 500 nm size are not differentiated, and existing LII techniques for determining small particle size are not satisfactory. Advantageously, the LII technique can provide instantaneous point measurements of soot concentration in turbulent flames such as are found in most practical combustors, including gasoline engines, Diesel engines, gas turbine engines, furnaces, and boilers.
It is desired to provide a LII method and apparatus suitable for both more accurately determining particle volume fraction and for determining primary particle diameter which is accurate, compact, transportable and suitable for use in situ for practical applications such as turbulent flame combustion devices, exhaust flow and ambient measurements.